KINESIOLOGICAL ELECTROMYOGRAPHY - Biomechanics and Motor Control of Human Movement - page 252

needle with an insulated conductor located inside and bared to the muscle
tissue at the open end of the needle; the needle itself forms the other con-
ductor. For research purposes, multielectrode types have been developed to
investigate the “territory” of a motor unit (Buchthal et al., 1959), which has
been found to vary from 2 to 15 mm in diameter. Fine-wire electrodes, which
have a diameter about that of human hairs, are now widely used. They require
a hypodermic needle to insert. After removal of the needle, the fine wires
with their uninsulated tips remain inside in contact with the muscle tissue.
A comparison of this experimental investigation of motor unit territory was
seen to agree with theoretical predictions (Boyd et al., 1978).
Indwelling electrodes are influenced not only by waves that actually pass
by their conducting surfaces but also by waves that pass within a few mil-
limeters of the bare conductor. The same is true for surface electrodes. The
field equations that describe the electrode potential were originally derived by
Lorente de No (1947) and were extended in rigorous formulations by Plonsey
(1964, 1974) and Rosenfalck (1969). These equationswere complicated by the
formulation of current density functions to describe the temporal and spatial
depolarization and repolarization of the muscle membrane. Thus, simplifica-
tion of the current density to a dipole or tripole (Rosenfalck, 1969) yielded
a reasonable approximation when the active fiber was more than 1 mm from
the electrode surface (Andreassen and Rosenfalck, 1981).
In the dipole model (see Figure 10.1), the current is assumed to be con-
centrated at two points along the fiber: a source of current, I , representing
Figure 10.1 Propagation of motor unit action potential wave front as it passes beneath
a recording electrode on the skin surface. The electrode voltage is a function of the
magnitude of the dipole and the distances r 1 and r 2 from the electrode to the depolarizing
and repolarizing currents.